This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present techniques. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present techniques. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Modern society is greatly dependent on the use of hydrocarbons for fuels and chemical feedstocks. Hydrocarbons are generally found in subsurface rock formations that can be termed “reservoirs.” Removing hydrocarbons from the reservoirs depends on numerous physical properties of the rock formations, such as the permeability of the rock containing the hydrocarbons, the ability of the hydrocarbons to flow through the rock formations, and the proportion of hydrocarbons present, among others.
Easily harvested sources of hydrocarbon are dwindling, leaving less accessible sources to satisfy future energy needs. However, as the costs of hydrocarbons increase, these less accessible sources become economically attractive. Recently, the harvesting of oil sands to remove bitumen has become more economical. Hydrocarbon removal from the oil sands may be performed by several techniques. For example, a well can be drilled to an oil sand reservoir and steam, hot air, solvents, or a combination thereof, can be injected to release the hydrocarbons. The released hydrocarbons may then be collected and brought to the surface. In another technique, strip or surface mining may be performed to access the oil sands, which can then be treated with hot water or steam to extract the oil. However, this technique produces a substantial amount of waste or tailings that must be disposed. Traditionally in the oil sand industry, tailings have been disposed of in tailings ponds.
Recent studies have been published that address the treatment of tailings as they are produced, in order to avoid the need for the large settling and storage areas. For example, International Patent Publication No. WO/2009/009887, by Bozak, et al., discloses a method for the recovery of tailings ponds. The method allows for treating tailings comprising a solids fraction and a hydrocarbon fraction. In the method, a primary flow is supplied to a jet pump. The primary flow includes water and less than 20% solids by mass. A secondary flow is supplied to a mixing chamber of the jet pump. The secondary flow includes a slurry of water and tailings, in which the slurry includes more solids by mass than the primary flow. The jet pump is operated using the primary flow such that the tailings are agitated to effect at least a partial phase separation of the hydrocarbon fraction from the tailings.
However, the recovery and treatment of these ponds may still add substantial costs to the production of hydrocarbons. Accordingly, processes that generate less waste may be useful. For example, one process for harvesting oil sands that generates less surface waste is the slurrified hydrocarbon extraction process. In the slurrified hydrocarbon extraction process, the entire contents of a reservoir, including sand and hydrocarbon, can be extracted from the subsurface via wellbore for processing at the surface to remove the hydrocarbons. The tailings are then reinjected via wellbores back into the subsurface to prevent subsidence of the reservoir and to allow the re-injected material to sweep the hydrocarbon bearing sands from the reservoir to the wellbore producing the slurry.
U.S. Pat. No. 5,823,631 to Herbolzheimer et al. discloses one such slurrified hydrocarbon recovery process that uses a slurry that is injected into a reservoir. In this process, hydrocarbons that are trapped in a solid media, such as bitumen in tar sands, can be recovered from deep formations. The process is performed by relieving the stress of the overburden and causing the formation to flow from an injection well to a production well, for example, by fluid injection. A tar sand/water mixture is recovered from the production well. The bitumen is separated from the sand and the remaining sand is reinjected in a water slurry.
International Patent Application No. WO/2007/050180, by Yale and Herbolzheimer, discloses an improved slurrified heavy oil recovery process. The application discloses a method for recovering heavy oil that includes accessing a subsurface formation from two or more locations. The formation may include heavy oil and one or more solids. The formation is pressurized to a pressure sufficient to relieve the overburden stress. A differential pressure is created between the two or more locations to provide one or more high pressure locations and one or more low pressure locations. The differential pressure is varied within the formation between the one or more high pressure locations and the one or more low pressure locations to mobilize at least a portion of the solids and a portion of the heavy oil in the formation. The mobilized solids and heavy oil then flow toward the one or more low pressure locations to provide a slurry comprising heavy oil, water and one or more solids. The slurry comprising the heavy oil and solids is flowed to the surface where the heavy oil is recovered from the one or more solids. The one or more solids are recycled to the formation, for example, as backfill.
The method discussed above converts the hydrocarbon bearing reservoir into a formation resembling a moving bed. When the reservoir moves toward the producer wells, void space is filled by the reinjected clean slurry stream. A critical aspect of the method is that this reinjected stream must have permeability that is higher than the relative permeability to water of the target formation. The slurry is not pushed, but rather dragged by the percolating fluid flow. Such methods may be considered a subset of a wider group of techniques used to inject tailings or wastes into subsurface spaces, such as mines and formations.
Backfill systems for reinjection of tailings in mining operations fall into two major flow categories. See Cooke, “Design procedure for hydraulic backfill distribution systems,” The Journal of The South African Institute of Mining and Metallurgy, March/April 2001, pp. 97-102 (hereinafter “Cooke 2001”). The first category is a free fall flow and the second category is a full flow or continuous flow.
The free fall flow systems are categorized by low flow rates such that gravity force is larger than friction force on a slurry, so that the slurry falls freely in the pipe until it reaches the free surface. The advantage of such a system is its tolerance to variations in tailings stream properties, such as solids volume concentration and flow rate. However, the backfilling pipes may often have a short life span. The reasons behind the short pipe life span include the impact damage of slurry freely falling with speed of up to 45 m/s, high impact pressure when slurry hits the free surface, high erosion rates when slight deviations from vertical occur in free fall region, and excessive pressure in the event of pipeline blockage.
The continuous flow systems are categorized by slurry occupying the full length of the reinjection well and the pipelines without any area of free fall. The advantage of this method is a much larger pipe life span as the free fall associated modes of pipe wear may be decreased. However, a fairly high backfill flow rate must be maintained so that friction loss is equal or greater than the backfill weight. Such systems may be sensitive to changes in flow rate and slurry rheology. Therefore, friction regulating/augmenting devices and techniques, such as liners, valves, breaks or, more often, solids volume concentration regulation are common. However, if the formation in the immediate vicinity of the injection represents a significant resistance to the backfill flow, then a large backpressure will develop which will support the weight of the backfill.
Most modern backfilling systems in mining operations are of the continuous type. Generally, hydraulic backfills are classified as slurries and pastes. See Cooke 2001. Slurries are characterized by low fraction of small particles or fines, for example, less than about 75 μm, and volume concentrations equal or lower than particle constant contact solid concentration, i.e., the volume concentration at or above which particles start developing permanent contacts with each other. Pastes, on the other hand, have large fines content and volume concentrations exceeding constant contact solid concentration, for example, about 45-50%.
The permeability of a slurry can be controlled by its water content, the average particle size and also the particle size distribution. See Mangesana, N., et al., “The effect of particle sizes and solids concentration on the rheology of silica sand based suspensions,” Journal of the Southern African Institute of Mining and Metallurgy, 108, 237-243 (2008). In particular, the smaller particles have the largest effect for the permeability control and, therefore, play a leading role into the design of any reinjection system. Several schemes have been suggested in the literature to address fluid rheology by particle size distribution or water content control. Previous art in this area is strongly related to particle size control and slurry distribution systems.
As suggested above, many efforts have been made previously in this area. Among the prior U.S. patents related to the technology disclosed herein, the following non-exclusive list is representative of those efforts: U.S. Pat. Nos. 3,508,407; 4,101,333; 5,141,365; 6,168,352; and 6,297,295.
These conventional prior systems for backfilling generally rely on an existing underground cavity. A borehole is drilled from the surface down to the underground cavity and fill material is then fed into the cavity either directly through the borehole or through a conduit placed in the borehole. It is often necessary to drill a number of such boreholes spaced a predetermined distance apart and to backfill through each of the holes to ensure that the underground cavity is filled as best is possible. The number of holes required is dependent upon the manner and degree to which the fill material is distributed in the cavity from the borehole. For example, in many systems, a slurry of fill material such as water and fine solids is merely deposited vertically into the cavity and is distributed in pyramidal fashion. This has been unsatisfactory in at least two respects. First, the slurry is not distributed very far laterally, thus a large number of boreholes are required. Second, after settlement of the slurry material, some top areas of the underground cavity remain unfilled and cave-ins continue to occur above those areas. Several improvements have been suggested in the literature. For example, U.S. Pat. Nos. 3,440,824; 3,608,317; 3,786,639; 4,968,187; and 6,431,796 are representative improvements.
In addition to the use of the backfill for mining purposes, numerous studies have focused on the use of backfill techniques to dispose of wastes. The disposal of the drill cuttings and drilling mud can be a complex environmental problem. Traditional methods of disposal include dumping, bucket transport, conveyor belts, screw conveyors, and washing techniques that require large amounts of water. Adding water creates additional problems of added volume and bulk, pollution, and transport problems. Installing conveyors may require major modification to a rig area and may involve extensive installation hours and expense.
Drilling waste injection or cuttings disposal into a subsurface formation has several advantages. For example no waste may be left on the surface. Transportation risks may be decreased or eliminated. There may be no liabilities for further clean-up once the disposal well is plugged. All of these advantages may improve the economics of a process. See Guo, Q. and Geehan, T., “An Overview of Drill Cuttings Reinjection—Lessons Learned and Recommendations,” 11th International Petroleum Environmental Conference, Albuquerque, N. Mex., Oct. 12-15, 2004.
Cuttings reinjection typically consists in a shearing and grinding system that converts the cuttings into a viscous slurry with the addition of water. The slurry is then injected by means of a high pressure pump, through hydraulic fracturing, into the subsurface using a well that extends relatively deep underground into a receiving stratum or adequate geological formation. The basic steps in the process can include the identification of an appropriate stratum or formation for the injection, preparing an appropriate injection well, formulation of the slurry, performing the injection operations, which may include fracturing the formation, and capping the well.
Related information on the reinjection of waste tailings may be found in U.S. Pat. Nos. 4,942,929; 5,129,469; 7,069,990; 7,730,996; 7,571,080; 5,310,285; 5,431,236; and 7,575,072 among many others. However, none of the techniques discusses the multiple simultaneous injections of a first slurry for harvesting subsurface materials and a second slurry that comprises a waste stream.